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As he coldly put it: "When I'm done, half of humanity will still exist. Perfectly balanced, as all things should be."

Comic books aside, scientists say human population growth is intangibly linked to the mass extinction event the group cited.

In fact, it's one of the largest ever recorded in the Earth's history, threatening a million species of plants and animals and often discussed by heavyweight naturalists Dame Jane Goodall and Sir David Attenborough.

Sir David Attenborough has been outspoken about the effect the population is having on the environment. Photo / Supplied

Dame Jane Goodall. Photo / Supplied

Comedian Ricky Gervais is a birth striker and has spoken about why he hasn't had children with partner Jane Fallon.

He explained on The Ellen DeGeneres Show in 2017: "There's loads of reasons why I don't love kids.

"The world is overpopulated. No one's sitting around going, 'Oh Rick is not having kids, we're going to run out.' There's loads."

It could take as long as 10 million years for the world's species diversity to recover from the mass extinction on our hands.

This so-called "recovery speed limit" has been observed throughout the Earth's geological history - right from the "Great Dying" that wiped out nearly all ocean life 252 million years ago to the massive asteroid strike that killed all non-avian dinosaurs.

And in the current case, Pepino was of course also correct to assert that the cause of it all was incontrovertibly us.

Comedian Ricky Gervais is a birth striker and has said "the world is overpopulated". Photo / Supplied

Scientists say that people - through burning and bulldozing habitats, over-exploiting resources and driving greenhouse gas emissions to unprecedented levels – are causing a "global spasm" of biodiversity loss.

This year, Yale University ecologists estimated that, over the next half century, human land use would reduce the habitats of 1700 species by up to 50 per cent, pushing them closer to extinction.

In few places was this as starkly apparent as New Zealand, where a third of our native land and sea birds have been pushed into oblivion, and more than 80 per cent are nearing the brink.

Pepino's call has since been answered by hundreds of people.

Kiwi researchers have found the environment does sometimes factor into peoples' deliberate decisions to remain childless.

Dr Tracy Morison, a lecturer at Massey University's School of Psychology, says such concerns are usually linked to a sense of moral responsibility.

"In my research, voluntarily childless people — and even those who are ambivalent about parenthood — often question what kind of world they would be bringing their child into and if it is a fair or moral action."

That's a fear that's been echoed by Conceivable Future, a US women's network dedicated to raising awareness around the threat that climate change poses to "reproductive justice".

Co-founder Josephine Ferorelli points to last year's UN report, warning there is little more than a decade to radically ramp down emissions to avoid a future climate more than 1.5C warmer.

"The data says there's a ticking clock," she told CNN.

"The 11-year window more or less approximates a lot of our reproductive windows as well."

"What kind of harm will a hotter and more painful world inflict on my child? Nobody has the answers for that."

Influential US congresswoman Alexandria Ocasio-Cortez recently posted on Instagram: "Is it still ok to have children?" Photo / AP

That same question was recently raised by influential US congresswoman Alexandria Ocasio-Cortez.

She posted on Instagram: "There's a scientific consensus that the lives of children are going to be very difficult - is it still ok to have children?"

One birth-striker, 25-year-old Alice Brown from Bristol in the UK, told Vice how that uncertainty did weigh on her mind.

"There are millions of people moving around the world, on the borderline of survival, already," she said.

"They are looking for a safe place to live, and look at the hostility they face. Think about the way we treat refugees at the moment – what hostility is going to exist in the world in the future? That's so scary and depressing for me. Our kids wouldn't be able to hide from that; that's the world that they are going to live in."

And projections tell us that the future indeed does appear scary.

By the middle of the century, when students taking part in this year's climate marches will be in their 40s, and with children of their own, average temperatures will be, at best, 0.5C warmer than today.

That means roughly 50 per cent more heatwaves, more droughts and more river floods.

At worst, temperatures will be about 1.5C higher than now, causing a tripling of heatwaves and droughts.

By the end of the century, assuming greenhouse gas emissions continue to climb close to current levels, many parts of our country will record more than 80 days a year above 25C.

Most places typically only have between 20 and 40 days above that now, yet, already, about 14 elderly people in Auckland and Christchurch die each year when the mercury climbs above 20C.

The world population, which hit seven billion in 2011, is growing by about 83 million each year. Photo / 123RF

If global temperatures climb just one, two or three degrees above current levels, that same death rate could rise to 28, 51 and 88 respectively.

And that was only what lay on the immediate horizon.

The best-case scenario is that global warming stops somewhere between 2050 and 2070.

The worst case is that global temperatures ended up about 4C above present by the close of the century – and sea levels rose 1.5m higher, with at least another 10m locked in across coming centuries.

Heavy rainfall and flooding events would quadruple, and extreme high temperatures would make many mid-latitude areas hard to bear in summer.

Leading Victoria University climate scientist Professor James Renwick, who spoke at a conference on population issues in Wellington this week, says the bigger increase there was in climate change, the harder it was to judge what would unfold.

"Really bad things could happen with unabated climate change, as we'd be living in a climate so far outside anything humanity has experienced before, and all bets would be off."

With dire warnings of this bleak potential future constantly stoking apocalyptic headlines, it was little wonder prospective parents did fear for their offspring.

Morison says while child-free peoples' anxiety about the future didn't just apply to the environment – financial security was another big issue - worries about climate change were indeed proving an increasing issue.

"On the other hand, some feel disinclined to have children because they feel that it would be morally wrong to bring another little consumer into the world," Morison says.

"They want to limit their personal impact on the planet."

In any case, her research suggested that, unlike Pepino, most people who refrained from having children didn't strongly wish to be a parent in the first place.

"Sustainability issues feature as another good reason not to procreate, rather than their primary or sole reason," she says.

"This may change as the environmental crisis is felt more keenly — as the advent of movements like birth strikers suggests."

Professor Alistair Woodward, a public health doctor and University of Auckland epidemiologist, says no one could dispute the logic that a rising population was bad for a planet already at breaking point.

Pepino is a member of the BirthStrike movement. Photo / This Morning

"All else being equal, more people means more consumption, resulting in greater environmental damage, including climate disruption."

He respects those who opted not to have children.

"We are fortunate to live in a society in which most people can choose where and when and how many children. Overseas this is often not the case."

In the most recent major assessment by the United Nations' Intergovernmental Panel on Climate Change (IPCC), it was estimated that global carbon dioxide emissions might be lowered by nearly a third if contraception was available to all women who expressed a need for it.

"On the whole these are women who want fewer children, later, and more widely spaced, but not necessarily no children at all."

Another 2017 study, published by Sweden's University of Lund and Canada's University of British Columbia, found the single most effective measure an individual in the developed world could take to cut their carbon emissions over the long term was to have one fewer child.

In fact, their study established that it was 25 times more effective than the next most effective measure - living without a car.

Dramatically, a group of environmental scientists argued last year that societies should embrace population ageing, as has been experienced here, and even decline.

They cited multiple reports of the socioeconomic and environmental benefits of population ageing, while pointing out smaller populations made for more sustainable societies.

"In many countries, stable and declining populations due to demographic ageing are often reported by the media as a problem or crisis," explains Frank Gotmark, a biologist at Sweden's University of Gothenburg.

"But the alternative - endless population growth - is not ecologically possible.

"Overpopulation leads to serious problems, including excessive consumption, deadly conflicts over scarce resources, and habitat loss leading to species endangerment."

A United Nations' population report from 2017 stated that 14 per cent of countries currently had declining populations - among them Japan, Estonia and the Czech Republic.

"Broadly speaking, part of the problem with climate change and other environmental issues is that there are an awful lot of people on the planet," Professor James Renwick says. Photo / File

The report projects that about a third of countries will have shrinking populations by 2050.

And smaller populations, they say, also mean less crowding, which can cut commute times, reduce stress, maintain green areas and improve quality of life.

"If we don't reverse overpopulation, what happens next will be a sad story," Gotmark says.

"We have to recognise that continued population growth is a global threat. Short-term economic concerns, while valid, cannot be prioritised over the long-term health of our environment and our societies."

That sentiment certainly rang true in the most recent report of the UN's IPCC, which warned high population growth would be a "key impediment" to hitting the critical target of limiting global warming to 1.5C.

Those nations with massive populations, like India and China, were among the most significant contributors to climate change overall, despite relatively lower impacts from each individual.

While India and China had relatively low population growth, it was expected that people born today in countries whose populations were still expanding rapidly would have a climate impact for generations to come.

Renwick says these rapidly-booming populations are often those expected to experience the worst impacts of climate change.

"That will put a lot of stress on communities least able to respond effectively."

The dramatic projections for developing nations also posed another worrying question - how will we feed all of these extra people?

If current income and consumption growth trends continued unabated, agricultural production would have to ramp up by 60 per cent to satisfy the expected increased demands for food and feed.

At the same time, climate change will bring further price volatility on staple foods and occasional crop failures in major growing areas.

Even in the US, researchers say that climate change, combined with population growth, was setting the stage for water shortages in parts of the country long before the end of the century.

In Asia, economic and population growth, on top of climate change, could lead to serious water shortages across a broad swathe of the region by as early as 2050.

Under a worst-case scenario, major crop failures could be happening several times per decade – and water and food scarcity could drive the movement of tens of millions of people.

This spells much greater scope for regional warfare over resources, putting millions of lives at risk, or at least displacing populations.

High temperature extremes would also be making areas in the tropics and near-tropics uninhabitable - or close to it - sending many of them here.

While New Zealand's population was relatively small, Renwick says there is a worry that the country might become seen as something of a safe haven for displaced populations, from the Pacific and probably from farther afield.

"We may face strong pressure to accept a lot of migrants over coming decades," he said.

"Plus, we have an obligation to help out Pacific neighbours as sea levels rise and the climate becomes more extreme."

"Broadly speaking, part of the problem with climate change and other environmental issues is that there are an awful lot of people on the planet."

The world population, which hit seven billion in 2011, is growing by about 83 million each year.

Projections show it ballooning to 8.6 billion by 2030, 9.8 billion by 2050 – and 11.2 billion by 2100.

When we add our farmed animals, humanity takes up some 90 per cent of the total animal biomass on Earth.

As there are only so many square metres of land, Renwick says, growth has to stop at some point.

"Reducing family size is ultimately something the we'll all have to do, as a global community."

It could be argued it is already happening here in New Zealand, but not for environmental reasons.

Population growth since 2013 has been dominated by net migration, rather than the number of births, which have been relatively steady at about 60,000 a year for the past six years, despite a decline in birth rates.

In other words, the number of births for every 1000 people is falling - but the growing population means total births remain at relatively high levels, after reaching a recent peak of almost 65,000 in the period from 2007 to 2010.

New Zealand's total fertility rate in 2017 was down to 1.8 births per woman, its lowest recorded level.

In any case, many environmentalists argue that focusing too much on population distracts us from tackling the root causes of the ecological crisis we've created.

The global network behind the most commonly-used indicator of our impact on the world, the ecological footprint, calculated humans were chewing up natural resources about 1.7 times faster than they could regenerate.

But if everyone lives like people in supposedly clean and green New Zealand, which had the 39th highest ecological footprint, we'd need about 2.8 Earths to sustain our consumption.

And while New Zealand accounts for just 0.17 per cent of the world's greenhouse gas emissions, Kiwis make the 21st biggest contribution on a per capita basis – and the fifth highest within other rich nations in the OECD.

"If New Zealand could shrink its per capita carbon footprint so that it fell in the bottom quartile of the OECD rather than in the top quartile, that would make a big difference," Woodward says.

"The ways in which we house, sustain and service a growing population will be hugely influential."

As it stood, he says, our urban centres aren't dense enough, and we farm at a high environmental cost.

Climate Change Minister James Shaw. Photo / Mark Mitchell

Climate Change Minister James Shaw points out there happens to be 18 countries that slashed their emissions between 2005 and 2015, even while their populations and economies grew over the same period of time.

"So while there's obviously a correlation between population growth and use of resources, particularly in wealthier countries which, like New Zealand, have the highest per capita emissions, the evidence shows that it is possible to decouple emissions growth from population and economic growth," he says.

"The main challenge lies in adopting new technologies and business models and in being far more efficient with resources, which New Zealand has, so far, been slow to get started on."

But he stops short of offering his thoughts on BirthStrike's extreme course of action.

"The decisions people make over whether or not to have children are deeply personal," he says.

"It is not for me to judge anyone over the decisions they make in that regard."

BY THE NUMBERS:

1.8:

New Zealand's total fertility rate in 2017 was down to 1.8 births per woman, its lowest recorded level.

11.2 billion: Estimated population of the world by the end of the century.

50%: Over the next half century, human land use would reduce the planet's habitats of 1700 species by up to 50 per cent, pushing them closer to extinction.

2.8: The number of Earths the world would need if everyone lived as unsustainably as New Zealanders.

25: A recent study found, for individuals living in the developed world, having one less child would be 25 times more effective at reducing their carbon footprint than living without a car.